I finally found the problem with my qpopper.
Here is the explanation. It would be helpful for some (I hope).
I followed the instructions in the how-to. (Thanks)
Then when I tried a connection I closed right away, so I checked the /var/log/system.log
and this is what I found:
May 11 17:30:36 localhost xinetd[325]: START: pop3 pid=724 from=xxx.xxx.xxx.xxx
May 11 17:30:36 localhost xinetd[724]: setgroups( 0, NULL ) failed: Invalid argument (errno = 22)
May 11 17:30:36 localhost xinetd[724]: Your system may require that 'groups = yes' be defined for this service: pop3
So, I opened /etc/xinetd.d/pop3 and inserted after user = root :
groups = yes
restarted xinetd from webmin and voila!
IT IS FINALLY WORKING.